1. Field of the Invention
The invention concerns a magnetic resonance marking system to mark a flowing medium in a marking region, of the type using a radio-frequency transmission device to generate marking radio-frequency signals and a marking radio-frequency transmission coil to emit the marking radio-frequency signals in the marking region. Moreover, the invention concerns a magnetic resonance system to generate magnetic resonance exposures of an examination region of an examination subject with such a magnetic resonance marking system. The invention also concerns a method to control a magnetic resonance marking system to mark a flowing medium in a marking region, in which method the marking radio-frequency signals are generated and emitted in the marking region. The invention also concerns a method to generate magnetic resonance exposures of an examination region of an examination subject in a magnetic resonance system in which a medium flowing into the examination region is marked beforehand in a marking region so that the medium is identified in magnetic resonance exposures of the examination region.
2. Description of the Prior Art
Magnetic resonance tomography has become a widespread technique to acquire images of the inside of the body of a living examination subject. In order to acquire an image with this method, i.e. to generate a magnetic resonance exposure of an examination subject, the body or the body part of the patient that is to be examined must initially be exposed to an optimally homogeneous static basic magnetic field (most often designated as a B0 field), which is generated by a basic field magnet of a magnetic resonance system. Rapidly switched (activated) gradient fields for spatial coding that are generated by gradient coils are superimposed on this basic magnetic field during the acquisition of the magnetic resonance images. Moreover, RF signals (for example a radio-frequency pulse or a radio-frequency pulse sequence) of a defined field strength are radiated with a radio-frequency antenna into the examination volume in which the examination subject is located. The nuclear spins of the atoms in the examination subject are excited by means of this RF field (most often designated as a B1 field) such that they are deflected out of their steady state, in which the spins are aligned parallel to the basic magnetic field, and precess around the direction of the basic magnetic field. For this purpose, the radio-frequency pulses must be radiated at the resonance frequency of the nuclear spins to be excited (known as the “Larmor frequency”), which depends on the magnetic field in which the atoms or molecules to be excited are located. The magnetic resonance signals that are thereby generated are received by radio-frequency reception antennas. The reception antennas can be either the same antennas with which the radio-frequency pulses are radiated, or separate reception antennas. The magnetic resonance images of the examination subject are reconstructed in a processor based on the received magnetic resonance signals. Each pixel in the magnetic resonance image is associated with a small physical volume (known as a “voxel”) of the subject, and each brightness or intensity value of each pixel in the image is linked with the signal amplitude of the magnetic resonance signal that is received from the corresponding voxel.
A groundbreaking development in conventional magnetic resonance imaging has been techniques in which the perfusion of marked blood in the brain is acquired with the aid of a magnetic resonance apparatus. The blood supply in any region of the brain can be determined by a subtraction of two images: one with marked blood and one without marking. Brain activities can therefore be depicted, or variations of the blood flow can even be revealed in pathological cases such as strokes. The observation of the perfusion of blood or other marked bodily fluids can also be meaningful in other organs in order to more easily detect pathological cases.
Conventionally, the marking of blood has typically been implemented by the use of exogenic contrast agents based on gadolinium or the like. In order to be able to avoid the administration of such contrast agents, some time ago a technique known as the “ASL technique” (ASL=Arterial Spin Labeling) was developed, which is used particularly in the examination of the brain. The arterial blood in a marking region (for example in the neck region of the patient) is thereby electromagnetically marked (or “labeled”) by special excitation of the nuclear spins in the blood (more specifically, the water component of blood) before it reaches an examination region (the brain, for example). An image is acquired after a certain period of time in which the blood marked in such a manner has become distributed in the brain.
As described above, a radio-frequency antenna is required for this purpose, with which the “normal” imaging radio-frequency signals required for the magnetic resonance acquisition are emitted into the examination region, for example the head region of the patient or test subject. The transmission antenna that serves to emit the imaging radio-frequency signals is also designated as an “imaging radio-frequency transmission antenna” or “imaging radio-frequency transmission coil” in the following. This imaging radio-frequency transmission antenna can be, for example, a “whole-body antenna” that is permanently installed in the magnetic resonance data acquisition unit that surrounds the examination space. However, it can also be a local antenna, for example a head coil that is placed on the patient like a helmet during the examination. In such examinations, it is thus possible to use the whole-body coil to emit the pulses and to use the head coil only to receive the magnetic resonance signals. In principle, however, the head coil can also be used to transmit the radio-frequency signals and to capture the magnetic resonance signals. In some magnetic resonance systems (for example Polestar by Odinmed, www.odinmed.com), basic field magnets and radio-frequency transmission antennas are fashioned and arranged so that they enclose only the head of the patient. A corresponding examination space or “field of view” of such a “head system” is thus markedly smaller than that of a “whole-body system”.
To apply the ASL technique, an additional radio-frequency transmission antenna (designated in the following as a “marking radio-frequency transmission antenna” or “marking radio-frequency transmission coil”) can be used that emits the aforementioned radio-frequency signals used for the marking. This marking antenna is typically directly arranged locally on the examination subject, advantageously as close as possible to a suitable artery of the patient. It is most often a relatively small radio-frequency transmission antenna.
The ASL technique functions very well in “whole-body” magnetic resonance systems in which both a marking region and an examination region are arranged in a magnetic field homogeneity volume. However, the ASL technique according to the prior art cannot be applied in magnetic resonance systems with a small homogeneity volume (for example in “head systems” of the aforementioned type). In such systems the homogeneity volume does not extend down to the carotid artery or the neck region of a patient, and thus the marking region lies outside of the magnetic field homogeneity region. Since the magnetic field strength outside the magnetic field homogeneity region declines quickly with increasing distance from the magnet or magnet system, the marking of the medium cannot take place with marking radio-frequency signals at the Larmor frequency that exists as a result of the basic magnetic field. Moreover, the imaging radio-frequency transmission antenna installed in the system cannot be used to mark the medium, because it does not extend into the neck region.